Well If you are talking about an AC component under AC conditions then the formula for the Capacitive Reactance is: XC = 1/2π f C, Im not sure which example you are talking about, but I'll go on anyway.If we are going to make the Resistance (R) which is in Parallel with Capacitor C, then the formula becomes;XC = 1/2π f C = R, then to determine the frequency f the formula is transposed to give; f = 1/2πRC, Think of it like this, as XC = R the voltages across each component will be equal, plotting this out on a Phasor diagram will show that the angle of VR and VC are 45 degrees , therefore the voltage VR is equal to Vin cos(45) =Vin/(root2) which is 0.707. Basically you need to draw phasor diagrams, using complex numbers will help.

i have never seen the formula for calculating cut-off, instead just know abt it tht it is the breakdown voltage for transistor , MOSFET . can u plz explain again tht why it is cut off at 0.7 why not more or less than tht

me tink it was 0.7071 wen i were in school a long time ago, look it up in a sine table[ 4 fig ] to reveal the hidden angle or inverse sine on a calculator, if i remember rightly we were fiddling with rms value